Structure and function of bone

3 types of long bone

-*diaphysis*/Shaft -*metaphysis*/flaring of the bone -*epiphysis*/ends of the bone

Osteoclast

-*responsible for bone breakdown* -extremely large and are derived from hematopoietic precursors -similar to monocytes and macrophages -*produce tartrate resistant acid phosphatase* -live in regions of bone resorption pits called * howship's lacunae* -have a ruffled border, which forms when they bind the surfaces of bone

metaphyseal complex

-Arise from periarticular plexuses that penetrate directly into the metaphyseal region

Clinical implications of the vascular supply to bone

-Bone needs healthy blood supply to heal fractures and revitalized damaged areas -*injury to the blood supply* either from fracture or iatrogenic* can cause delays in healing* -bones with more robust blood supply = femur and humerus = quick healing and more predictable healing

Blood supply to the bone

-Critical for maintenance, healing, and function

Osteoblasts

Bone forming cells -drive signaling for bone breakdown -make collagen type I -produce osteocalcin and various other extracellular matrix proteins - produce alkaline phosphatase -derived from marrow stromal cell or mesenchymal stem cells

Mechanical properties of cortical bone

More stiff than cancellous bone -has maximum strength and stiffness for* axial loads*

Mechanical properties of trabecular bone

Much* lower stiffness and strength* -able to absorb a large amount of energy and allows larger deformations before failure -*more compliant *than cortical bone -*important for function around joints*

3 major bone cell types

Osteoblast, osteoclast, osteocyte

Responsible for basic coupling or formation degradation signaling

Osteoblasts

osteocytes

Osteoblasts that become surrounded by bone matrix -derived from marrow stromal cell or mesenchymal stem cells/pluripotent marrow cells -most numerous of bone cells -characterized by extensive canaliculi that establish communication between adjacent osteocytes and the canals the a gap junctions

Nutrient arteries

Pass directly into the diaphysis through nutrient foramen in the diaphyseal cortex entering the intramedullary canal -*supply the inner two thirds of the diaphyseal cortex*

Bisphosphonates

Prescribed for osteoporosis and are incorporated into bone by osteoblasts -synthetic analogs of inorganic phosphate -once placed in bone they disrupt bone degradation by disrupting the ruffled border of the osteoclasts or by causing apoptosis of the osteoclasts

3 main sources of blood supply to long bones

-Nutrient arteries -arteries in the periosteal membrane -metaphyseal complex

osteocyte function

-Participate in signaling in communicating the various loads and strains in the bone to guide formation and degradation

osteon

-also called Haversian system -cylindrical structures 3-6 mm long and 150-200 µm in diameter -*center contains blood vessels and poorly myelinated nerve fibers* -surrounding lamellae contain osteocytes

Percent of body sodium and magnesium stored by bone

65%

Percent of body's phosphorus stored by bone

85%

Percent of body's calcium stored by bone

98-99%

makes RANK-ligand and OPG

????

Butterfly

Area in which pressure was applied

Quick healing properties of bone such as the femur and humerus

Attributed to larger soft tissue envelope of highly vascular muscle tissue surrounding the bone

axiallyLoading bone

Sheer type fracture

Continuous exposure to PTH

Signals bone degradation by working to the osteoblast

act at the osteoblast level

Vitamin D parathyroid hormone interleukin 1 prostaglandins

porosity of trabecular bone

between 30-90%

osteoblast

cell that makes bone -master regulators of cell turnover, bone making and breaking -arise from MSC cells

Osteoprotegerin (OPG)

competitive inhibitor of RANK ligand

nerves in long bone

enter from the periosteum alongside the arteries and are found along the blood vessels and bone in the bony canal system

osteocytes

make up 90% of bone cells -quiescent -function is unclear; calcium homeostasis? -MSC origin

Prostate cancer

rankl <<< OPG

osteoclast

remove bone ruffled border come from osteopoeitic stem cells

osteon/haversian system

single blood vessel unmyelinated bone

Simple fractures

spiral oblique Simple

Bending bone

tension type fracture

cancellous

trabecular

oblique fracture

uneven bending

Bones with much *longer healing times* that are more predisposed to *non-unions*

*Tibia* → largely subcutaneous * talus* → largely covered in a vascular cartilage

Point at which force is applied to butterfly fractures

-Force/bending occurs adjacent to the butterfly -transverse fracture occurs on the tension side

trabecular bone

-Found primarily in the metaphyseal region of long bones -described as a collection of plates and or rods -structures have a maximum thickness of approximately 200 µm -space between the struts of the trabeculi make the bone porous and allow it to be filled with marrow -architecture designed to *maximize strength and minimize weight*

3 types of bone in the human body

-Long bones → femur, tibia, humorous -small or short bones → vertebra, sternum, carpal bones and tarsi -flat bones → skull, scapula, pelvis, and mandible

cortical bone

-compact bone -extremely dense with very low porosity -thickwalled cylinder surrounded by intramedullary canals -*acts as a loadbearing segment* -arranged into *osteons*

Intra-membranous ossification

-flat bones -form through loose condensation's of mesenchymal tissue, which give rise to bone directly

bone composition

-inorganic matrix → 60-70% -collagen → about 90% of the organic matrix -non-collagenous protein → 8% -water → 5-8%

Endochondral ossification

-long and short bones -formation starts with a collagenous model which is then replaced by bone

bone loaded for failure

-rarely fails in compression -more likely to fail in tension or shear

periosteal membrane arteries

And to the bone at various points and supply the outer 1/3rd of the cortex of the diaphysis

Comminuted fractures

Can be caused by - high-speed torsion -multiple points of bending - crush

Intermittent exposure to PTH

Causes bone formation -principal mechanism used in the drug Forteo prescribed for osteoporosis

3 forces acting on bone

Compression, tension, and shear bone is strongest against compression >tension >sheer

Transverse fracture

Pure bending

Turns on osteoclast activity when signaled by osteoblasts

RANK ligand

normal stromal cells

RANKL = OPG

Giant cell tumors

RANKL > opg

Breast cancer

RANKL >> opg

Multiple myeloma

RANKL >> opg

spiral fracture

Torsion

2 major tissue organizations of mature bone

Trabecular/cancellous bone cortical bone

Butterfly fractures

Usually result from bending and torsion or bending and compression

Osteocalcin

Very specific marker for osteoblast activity

cortical/compact

stiffer, stronger less forgiving